Operation Alamo starts tomorrow, December 14, and ends on December 17, according to the BioWare blog.

It challenges players with earning a special "Survived 5 Waves" medal on the Firebase Condor map. This means that they'll have to stay alive for five consecutive waves in order to unlock it and earn the special Commendation Pack reward.

You can complete the weekend challenge at any difficulty and you don't have to extract in order to finish it.